Routers with Very Small Buffers

Yashar GanjaliStanford University

Joint work with: Mihaela Enacescu, Ashish Goel, Nick McKeown, and Tim Roughgarden

Presented byArjumand Younus, 20093649

Outline (1/2) Background and Problem Statement Motivation The Router Buffer Story How Much Buffering Do We Need? Single TCP Flow Many TCP Flows Buffer Size – Theory vs. Practice Small Buffers Scenario

Yashar Ganjali Routers with Very Small Buffers, INFOCOM 2006 2

Outline (2/2) Intuitive Explanation of O(log W) Buffer Size

– Leaky Bucket TCP Reno Paced TCP Simulations with O(log W) Buffers Conclusion

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Background and Problem Statement Congestion Control Buffering - first component of any congestion

control solution. Buffers ensure that link is utilized 100%. The Problem:

How much buffering? – Is sparking much debates recently.

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Motivation - Networks with Little or No Buffers (1/2) Problem

Internet traffic is doubled every year Disparity between traffic and router growth

(space, power, cost) Possible Solution

All-Optical Networking Consequences

Large capacity large traffic Little or no buffers

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Motivation - Why Does Buffer Size Matter? (2/2) End to end latency:

Transmission delay Propagation delay Queuing delay

Buffers are costly. 1/2 board space of routers 1/3 power consumption

Small buffers: On chip higher density Lower cost

The only variable component of latency

The Story

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)(logO2

2 )2()1( Wn

CTCT

)(logO

22 )2()1( W

n

CTCT

(1) Assume: Large number of desynchronized flows; 100% utilization(2) Assume: Large number of flows; <100% utilization

1,000,000 10,000 20# packetsat 10Gb/s

SawtoothPeak-to-trough

Smoothing of many sawtooths

Non-bursty arrivals

Intuition& Proofs

Simulated ManyTCP Flows

Evidence

Routers with Very Small Buffers, INFOCOM 2006

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How Much Buffering Do We Need?

Universally applied rule-of-thumb: A router needs a buffer size:

2T is the two-way propagation delay (or just 250ms) C is capacity of bottleneck link

Context Mandated in backbone and edge routers. Appears in RFPs and IETF architectural guidelines. Usually referenced to Villamizar and Song: “High Performance

TCP in ANSNET”, CCR, 1994. Already known by inventors of TCP [Van Jacobson, 1988] Has major consequences for router design

CTB 2

CSource Destination

2T

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Rule for adjusting W If an ACK is received: W ← W+1/W If a packet is lost: W ← W/2

Single TCP Flow

Only W packets may be outstanding

Routers with Very Small Buffers, INFOCOM 2006

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Rule for adjusting W If an ACK is received: W ← W+1/W If a packet is lost: W ← W/2

Single TCP Flow

Only W packets may be outstanding

Source Dest

maxW

2maxW

t

Window size

CT 2

CT 2

Routers with Very Small Buffers, INFOCOM 2006

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ProbabilityDistribution

B

0

Buffer Size

W

Many TCP Flows

Routers with Very Small Buffers, INFOCOM 2006

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Smooth Traffic - Theory Theory: For smooth traffic very small buffers

are enough. Poisson Traffic

Loss independent of link rate, RTT, number of flows, etc.

Can we make traffic look “Poisson-enough” when it arrives to the routers…?

Can we make traffic look “Poisson-enough” when it arrives to the routers…?

M/D/1Poisson

BD

B loss

%1loss pkts20 80%, .. Bei

Large Buffers - Practice

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Typical OC192 router linecard buffers over 1,000,000 packets

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Small Buffers with Paced Injections Assume:

Buffer Size > Distance between consecutive packets of a

single flow S > Limited injection rate

Flows are not synchronized Start times picked randomly and independently

tWi

t

Win

dow

siz

e

RTT

tWPoisson iTransmit

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Small Buffers – Realistic ScenarioAssumptions:

Internet core is over-provisioned Example: Load < 80%

There is spacing between packets of the same flow: Natural: Slow access links Artificial: Paced TCP

Result:Traffic is very smooth, and loss rate is very low,

independent of RTT, and number of flows.

With a buffer size of about 20 packets we can gain high throughput.

With a buffer size of about 20 packets we can gain high throughput.

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Leaky Bucket – Paced vs. RenoBucket drains with a constant rate. Load is 90% for both cases.

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TCP RenoTCP Reno sends packets in a burst High drop rate

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Paced TCPSpacing packets Much lower drop rate

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Simulations with O(log W) Buffers Regular TCPRegular TCP

TCP WithPacing

TCP WithPacing

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Simulations with O(log W) Buffers

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Simulations with O(log W) Buffers

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Conclusion Very small buffers are OK if:

Sacrifice 10-20% throughput Pacing: natural, or TCP modification

Major consequences for electronic routers: Board space reduction Power reduction Increased density

Opens doors to all-optical networking. Experimental validation is in progress.

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Thank You!Thank You!

Questions?Questions?